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The latest brain teaser sweeping across the internet isn’t one of those 3D puzzles or an old image that has hidden pictures within it. No. The latest puzzle that has gone viral involves what looks like a high school geometry lesson. But instead of boring people like their 10th grade math teacher did, this puzzle is getting people’s brains buzzing. While the task sounds simple enough, executing it is not easy. You’re supposed to identify how many triangles are in the image. With a simple line illustration like this, how hard can it be? You better look twice because most people are getting it wrong on their first try.

Earlier this month the image was shared to Quora. Even there, where a lot of smart people congregate, users were scratching their heads trying to identity how many triangles are hidden within the image.

Take a look at the drawing here. Someone simply took a piece of paper and drew the diagram on it with a ruler and a pen. The image looks simple. It includes a large triangle with other lines dissecting it throughout. The lines connect to form a grid and divide the image into various triangles. This makes it more complicated when trying to calculate how many are in the image.

You can count the triangles one by one and mark them off as you do. Or if you’re good at math and remember your skills, you can simply do math to figure it out. Because each row contains six triangles and there are four line columns you can simply multiply six by four and get twenty-four.

Most users on Quora agreed that there are 24 triangles in the in the image. But only an astute, careful observe would see the 25th triangle hidden within the image.

Check out the video below to take this test for yourself!

It’s in the artist’s signature! Yes, this is trick puzzle that has infuriated a lot of people online, who spent time counting out the triangles. Almost everyone missed the final triangle hidden in the signature.

For those who had trouble calculating even twenty-four, mathematician Martin Silvertant created a simple diagram to show everyone where the triangles were. But even Silvertant missed the 25th triangle hidden inside the signature.

Readers online are outraged that the 25th is hidden in plain sight. Here were comments shared on NDTV.com:

“Signature? Seriously? Why don’t we also count the word triangle and call it 26?”

“24 Simply. There are 4 horizontal lines as base. At each base 5 triangles made. Plus 4 big triangles. 5×4 + 4 = 24”

“Signature does not count.”

“Simply something. Foolish 25th.” Sorry, but it does count!

Do you agree that the 25th triangle in the signature should count or not? Is that an unfair trick for people to have to uncover? Most people think it is. And that is the main reason the internet is furious about the puzzle.

How many triangles did you count when you did it? Did you see the last one?